The present invention relates to processes for converting lower molecular weight alkanes to olefins, higher molecular weight hydrocarbons, or mixtures thereof that may be useful as fuels or monomers and intermediates in the production of fuels or chemicals, such as lubricants and fuel additives, and more particularly, in one or more embodiments, to processes wherein a gas that comprises lower molecular weight alkanes is reacted with bromine to form alkyl bromides and hydrobromic acid in a manner to effectively reduce formation of multi-brominated species to a level that can be tolerated in subsequent process steps.
Natural gas which is primarily composed of methane and other light alkanes has been discovered in large quantities throughout the world. Many of the locales in which natural gas has been discovered are far from populated regions which have significant gas pipeline infrastructure or market demand for natural gas. Due to the low density of natural gas, transportation thereof in gaseous form, for example, by pipeline or as compressed gas in vessels, is expensive. Accordingly, practical and economic limits exist to the distance over which natural gas may be transported in its gaseous form. Cryogenic liquefaction of natural gas (often referred to as “LNG”) is often used to more economically transport natural gas over large distances. However, this LNG process is expensive and there are limited regasification facilities in only a few countries that are equipped to import LNG.
Another use of methane is as feed to processes for the production of methanol. Methanol can be made commercially via conversion of methane to synthesis gas (CO and H2) (often referred to as “syn-gas”) at high temperatures (e.g., approximately 1000° C.) followed by synthesis at high pressures (approximately 100 atmospheres). There are several types of technologies for the production of synthesis gas from methane. Among these are steam-methane reforming (SMR), partial oxidation (POX), autothermal reforming (ATR), gas-heated reforming (GHR), and various combinations thereof. There are advantages and disadvantages associated with each. For instance, SMR and GHR usually operate at high pressures and temperatures, generally in excess of 600° C., and are endothermic reactions thus requiring expensive furnaces or reactors containing special heat and corrosion-resistant alloy heat-transfer tubes filled with expensive reforming catalyst and high-temperature heat supplied from a source external to the reactor, such as from the combustion of natural gas, as is often utilized in SMR. POX and ATR processes usually operate at high pressures and even higher temperatures, generally in excess of 1000° C. and utilize exothermic reactions in which a significant fraction of the hydrocarbon feed is converted to CO2 and a large amount of high-temperature waste-heat must be rejected or recovered, thus complex and costly refractory-lined reactors and high-pressure waste-heat boilers to quench and cool the synthesis gas effluent are required. Also, significant capital cost and large amounts of power are required for compression of oxygen or air to these high-pressure processes. Thus, due to the high temperatures and pressures involved, synthesis gas technology is generally viewed as expensive, resulting in a high cost methanol product. This cost can limit higher-value uses of the methanol, such as for chemical feedstocks and solvents. Furthermore, it is generally thought that production of synthesis gas can be thermodynamically and chemically inefficient, in that it can produce large excesses of waste heat and unwanted carbon dioxide, which lowers the carbon conversion efficiency of the process. Fischer-Tropsch Gas-to-Liquids (GTL) technology can also be used to convert synthesis gas to heavier liquid hydrocarbons, however investment cost for this process at this point in time are higher than other types of processes. In each case, the production of synthesis gas represents a large fraction of the capital costs for these methane conversion processes and limits the maximum carbon efficiencies that these processes can attain.
Numerous alternatives to the conventional production of synthesis gas as a route to methanol or higher molecular weight hydrocarbons have been proposed. However, to date, none of these alternatives has attained commercial status for various reasons. Some of the previous alternative prior-art methods are directed to reacting a lower alkane, such as methane, with a metal halide to form an alkyl halide and hydrogen halide, which can be reacted with magnesium oxide to form corresponding alkanols. Halogenation of methane using chlorine as the halogen usually results in poor selectivity to the monomethyl halide (CH3Cl), but rather produces unwanted by-products such as CH2Cl2 and CHCl3. These are thought to be difficult to convert or require severe limitation of conversion per pass, and hence very high recycle rate.
Other existing processes propose the catalytic chlorination or bromination of methane as an alternative to generation of synthesis gas (CO and H2). To improve the selectivity of a methane halogenation step in an overall process for the production of methanol, one process teaches the use of bromine, generated by thermal decomposition of a metal bromide, to brominate alkanes in the presence of excess alkanes, which results in improved selectivity to mono-halogenated intermediates such as methyl bromide. To avoid the drawbacks of utilizing fluidized beds of moving solids, the process utilizes a circulating liquid mixture of metal chloride hydrates and metal bromides. Other processes are also capable of attaining higher selectivity to mono-halogenated intermediates by the use of bromination. The resulting alkyl bromides intermediates such as methyl bromide, are further converted to the corresponding alcohols and ethers, by reaction with metal oxides in circulating beds of moving solids. Another embodiment of such processes avoids the drawbacks of moving beds by utilizing a zoned reactor vessel containing a fixed bed of metal bromide/oxide solids that is operated cyclically in four steps. While certain ethers, such as dimethylether (DME) are a promising potential diesel engine fuel substitute, as of yet, there currently exists no substantial market for DME, and hence an expensive additional catalytic process conversion step would be required to convert DME into a currently marketable product. Other processes have been proposed which circumvent the need for production of synthesis gas, such as U.S. Pat. No. 4,467,130 to Olah in which methane is catalytically condensed into gasoline-range hydrocarbons via catalytic condensation using superacid catalysts. However, none of these earlier alternative approaches have resulted in commercial processes.
In some instances, substituted alkanes, in particular methanol, can be converted to olefins and gasoline boiling-range hydrocarbons over various forms of crystalline alumino-silicates also known as zeolites. In the Methanol to Gasoline (MTG) process, a shape selective zeolite catalyst, ZSM-5, is used to convert methanol to gasoline. Coal or methane gas can thus be converted to methanol using conventional technology and subsequently converted to gasoline. However due to the high cost of methanol production, and at current or projected prices for gasoline, the MTG process is not considered economically viable. Thus, a need exists for an economic process for the conversion of methane and other alkanes found in various gas feeds to olefins, higher molecular weight hydrocarbons or mixtures thereof which have an increased value and are more economically transported thereby significantly aiding development of remote natural gas reserves. A further need exists for an efficient manner of brominating alkanes present in various gas feeds, such as natural gas, to mono-brominated alkanes while minimizing the amount of undesirable multi-halogenated alkanes formed.